Enceladus has a density of 1.6 g/cc, so it is mostly water and has little of the heavy radioactive elements. It probably does have a noticeable amount of light potassium-40 though. Earth has a density of 5.5 g/cc, because it is loaded with the dense elements, especially the uranium and thorium that make much of Earth's geothermal heat.
(Numbers from Wikipedia.)